Communications traffic is expected to increase rapidly due to the spread of new services such as cloud computing, video distribution using the Internet, and the like. In order to cope with the increase in communications traffic, research and development of optical transceivers that can transmit 100 Gbps-order signals are carried out.
However, an increase in bit rate per wavelength leads to a greater deterioration in signal quality due to lowered tolerance with respect to the Optical Signal to Noise Ratio (OSNR), wavelength dispersion in transmission channels, and waveform distortion caused by polarization mode dispersion or by a nonlinear effect. Accordingly, digital coherent receiving methods yielding the OSNR tolerance and the waveform distortion tolerance in transmission channels have been attracting attention in recent years (non-Patent Document 1).
In optical digital coherent receiving methods, a high quality can be attained even at high bit rates because these methods make it possible to perform waveform distortion compensation via the improvement of the OSNR tolerance and digital signal processing circuits, and to perform adaptive equalization in response to temporal variations of the propagation characteristics of the optical transmission channel.
By contrast to a conventional method in which signal intensities are expressed by an ON or OFF assigned to binary signals and waves are detected directly, in an optical digital coherent receiving method, the light intensity and the phase information are extracted using a coherent receiving method, and the extracted intensity and phase information are quantized by using an Analog-Digital Converter (ADC) in order to perform demodulation using a digital signal processing circuit.
DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying), a phase modulation method used for optical digital coherent receiving methods, is capable of assigning two-bit data to four modulated optical phases (0 deg, 90 deg, 180 deg, and 270 deg) for the P polarization and the S polarization, respectively. In DP-QPSK, the symbol rate can be reduced to one-fourth of the information transmission rate, making it possible to reduce the size and cost of the systems.
FIG. 1 illustrates an exemplary configuration of a conventional optical digital coherent receiver.
In an optical digital coherent receiver 10, an ADC 12 performs a quantization process on received optical data, and a received data digital processing unit 11 digitally performs a process after the quantization process.
An imbalanced amplitude correction unit 13 in the received data digital processing unit 11 corrects the imbalance between the I signal amplitude and the Q signal amplitude of the received data containing the I signal and the Q signal that have been digitized by the ADC 12. This correction is performed because a great imbalance between the I signal amplitude and the Q signal amplitude will often cause errors in processes to be executed later. Signals processed by the imbalanced amplitude correction unit 13 are input to a fixed equalizer 14. The fixed equalizer 14 digitally performs dispersion compensation and compensation (equalization) for waveform deformation caused by polarization mode dispersion or by a nonlinear effect, or the like. The fixed equalizer 14 performs a quantization process for a prescribed quantity. Thereby, dispersion caused by characteristics unique to each optical transmission channel is compensated for. Signals output from the fixed equalizer 14 are input to a sampling phase adjusting unit 15. The sampling phase adjusting unit 15 adjusts a timing at which the signal value of a received signal is sampled, and adjusts a sampling timing on the basis of the sampling phase value detected by a sampling phase detection unit 16. When a sampling timing coincide with the transitioning timing of a signal, the sampled value of the signal tends to be erroneous, and thus attention is paid so that the sampling timing does not coincide with the transitioning timing of a signal.
Signals output from the sampling phase adjusting unit are input to an adaptive equalizer 17. The adaptive equalizer 17 compensates for waveform distortion that was caused by aging degradation of transmission channels and that was not completely compensated for by the fixed equalizer 14. The adaptive equalizer 17 includes digital filters and controls the coefficients of tap coefficients so as to compensate for waveform distortion. As a tap coefficient, a value having a weight calculated by an equalization weight calculation unit 18 is set. Signals output from the adaptive equalizer 17 are input to a frequency offset estimation/compensation unit 19. The frequency offset estimation/compensation unit 19 compensates for the difference between the frequency of carrier waves used by the transmission side of the optical transmission system and the frequency of the station-transmitted waves used by the receiving side. When there is a difference between the frequency of carrier waves used by the transmission side and the frequency of the station-transmitted waves of the receiving side, the signal point on the I-Q plane turns on the I-Q plane, and thus such a difference is compensated for.
Output signals from the frequency offset estimation/compensation unit 19 are input to a carrier phase offset estimation/compensation unit 20. The carrier phase offset estimation/compensation unit 20 compensates for an offset of 90 degrees of the signal point on the I-Q plane. In other words, the frequency offset estimation/compensation unit 19 prevents the signal point from turning on the I-Q plane, and thereby the signal point stays at certain positions on the I-Q plane. However, even if the signal point is prevented from turning, there is still a possibility that the phase of the signal point will be at the position after the 90-degree turn of the phase of the signal point. Accordingly, a process is executed to turn 90 degrees backward to the original position a signal point that may be at the position of having been turned 90-degrees. This phase difference of 90 degrees is caused by an offset of phases of carrier waves, and thus, compensation for the phase offset of carrier waves makes it possible to return the signal point back to the original position.
Signals output from the carrier phase offset estimation/compensation unit 20 are input to an error correcting code unit 21. The error correcting code unit 21 performs Viterbi decoding or Turbo decoding, and evaluates which quadrant on the I-Q plane a signal point is likely to exist in by using a likelihood based method in order to correct errors.
Arts related to the present invention have been disclosed as below.
For example, there is an optimum threshold value setting circuit system for a discriminator of an optical receiver that responds to fluctuations of input signal levels, again control system having a wide dynamic range that utilizes a full scale range of an AD converter, an AGC amplifier for an optical receiver that responds to pulse signals, and a circuit system for protecting a photodetector and avoiding amplifier saturation that responds to pulse signals in an optical receiver.
Patent Document 1: Japanese Laid-open Patent Publication No. 2000-59309    Patent Document 2: Japanese Laid-open Patent Publication No. 2009-206968    Patent Document 3: Japanese Laid-open Patent Publication No. 10-173456    Patent Document 4: Japanese Laid-open Patent Publication No. 2005-39860    non-Patent Document 1: D. Ly-Gagnon, IEEE JLT, vol. 24, pp. 12-21, 2006
In the adaptive equalizer (AEQ) of an optical digital coherent receiver, the levels of signal components in adaptive-equalizer output signals vary depending upon the applied algorithm. In an error correcting process executed after an adaptive equalization process, the signal components of input signals are required to be at an optimum level (that is set by the circuit configuration of the error correcting code unit) for the error correcting process. However, a method of adjusting signal components of adaptive-equalizer output signals to an optimum level in an optical digital coherent receiver has not been disclosed.
As a conventional method of detecting a signal level, there is a method in which the average value of the total electric power of adaptive-equalizer output signals is used, and a method in which the average value of the peak electric power is used. In the method in which the average value of the total electric power of adaptive-equalizer output signals is used, if an adaptive-equalizer algorithm that makes the total electric power value of signal components and noise components a certain level is used, the average value of the total electric power of adaptive-equalizer output signals includes noise components, and thus variation in the level of signal components of adaptive-equalizer output signals caused by noise components included in the average value of the total electric power of adaptive-equalizer output signals prevents precise detection or adjustment of the level of signal components.
FIGS. 2A and 2B illustrate examples of adaptive-equalizer outputs having different amounts of noise in adaptive equalizer inputs (i.e., OSNRs).
When an adaptive equalizer algorithm that makes the total electric power of signal components and noise components of adaptive-equalizer output signals a certain level is used, the signal components vary depending upon the noise components. In FIG. 2, when the OSNR is 100 db (noise components: low as in FIG. 2A), the average of the signal components is approximately 1.0, while when the OSNR is 12 db (noise components: high as in FIG. 2B), the average of the signal components is smaller than 1.0, meaning that the levels of the signal components vary depending upon the noise components.
FIGS. 3A and 3B illustrate problems in adaptive-equalizer outputs caused by differences in the amounts of noise in adaptive equalizer inputs.
As illustrated in FIG. 3A, amplitudes of signal points in outputs from the adaptive equalizer when the OSNR is 100 dB are collected around a particular point, and the amplitude value is approximately 1.0. When the OSNR is 12 dB as illustrated in FIG. 3B, many noise components are included in the signal components, and accordingly the signal points spread and the average of the amplitude value of the signal components is approximately 0.85. In other words, when many noise components are included in the signal components of inputs of the adaptive equalizer, the amplitude of the signal components of outputs of the adaptive equalizer is affected, causing differences in the level of signal components between when the OSNR is excellent and the OSNR is poor. In addition, amplitude value r is expressed as r=√(I2+Q2) where I represents the amplitude of I signal and Q represents the amplitude of Q signal. In this expression, I2+Q2 represents the electric power value, and by calculating the root of this value, the amplitude value can be obtained. This relationship between the amplitude value and the electric power value is also applied to the explanations below.